Capillary chromatography, based on either liquid chromatography (LC) or gas chromatography (GC), is advantageous over conventional analytical-scale chromatography in many aspects. For example, capillary chromatography utilizes a low sample volume, low consumption of solvent and is capable of trace analysis. Capillary chromatography, when coupled with mass spectrometry, for example, capillary LC coupled with electrospray ionization-mass spectrometry (ESI-MS) and capillary GC with electron ionization mass spectrometry (EI-MS), results in good MS detection sensitivity because the low flow rate of capillary LC or GC is compatible with the ion source input rates of ESI-MS and EI-MS.
LC and GC differ in a number of ways, such as mobile-phase viscosity, average diffusion coefficient of a sample in the mobile phase and mobile-phase compressibility. One significant difference is that GC generally is used with samples which are volatile, can be evaporated intact at high temperatures or from which volatile derivatives can be reliably obtained. Consequently, GC devices typically operate at substantially higher temperatures than LC devices.
Despite many advantages of capillary LC and GC, chromatographers tend to make use of analytical-scale chromatography systems whenever possible because both capillary LC and GC also present difficult challenges. As an example, fused-silica tubes are commonly used in a capillary chromatography system due to their desirable features. The dimensions of fused silica tubing can be easily controlled during manufacturing. Moreover, the wall of a fused-silica tube is clean, non-reactive and smooth, providing good transport of small volumes of fluids. Unfortunately, fused-silica tubes are small, fragile and brittle. Thus an operator must be experienced and exercise due care to properly make fluidic connections using fused-silica tubes.
Capillary LC or GC systems operate at low flow rates, typically micro-scale or nano-scale flows, thus what would otherwise be considered leaks or void volumes can still degrade system performance parameters significantly, including chromatographic resolution and detection sensitivity. Moreover, the higher operational temperatures (e.g., greater than 300° C.) of capillary GC systems can induce deformation of fluidic system components and thus such systems are more susceptible to fluidic leaks.